1. Field of the Invention
The present invention pertains generally to photovoltaic system charge controllers and, more particularly, to photovoltaic system charge controllers that employ maximum power point tracking.
2. Brief Discussion of the Related Art
Photovoltaic (PV) systems that produce electricity from solar energy have established themselves as a successful and reliable option for electrical power generation. Photovoltaic systems have continually been gaining in popularity as the cost of such systems has been reduced, as the cost of utility-supplied power has escalated, and as greater attention has been paid to the need for renewable, alternative energy sources. Basically, a photovoltaic system includes a photovoltaic (PV) array made up of one or more PV panels or modules composed of photovoltaic cells capable of converting solar energy into electrical energy, a battery bank made up of one or more batteries for storing the electrical energy produced by the photovoltaic array, and a charge controller for controlling the charging of the one or more batteries with the electrical energy produced by the photovoltaic array. The electrical energy stored in the battery bank is available to power a load, and inverters are sometimes used to convert the battery direct current (DC) into alternating current (AC) suitable for AC loads. Photovoltaic systems are frequently employed to power loads independently of utility power, such as where electrical power from a utility grid is unavailable or not feasible, and these photovoltaic systems are commonly referred to as “off-grid” and “stand-alone” photovoltaic systems.
Photovoltaic systems have been designed with traditional charge controllers that do not employ maximum power point tracking (MPPT), and such controllers may be referred to as non-MPPT charge controllers. Non-MPPT charge controllers connect the PV array directly to the battery bank for charging. Usually there is a mismatch between the output voltage of the PV array and the voltage required to charge the battery bank that results in under-utilization of the maximum power output from the PV array. The reason for the mismatch is that most PV modules are rated to produce a nominal 12V under standard test conditions but, because they are designed for worse than standard test conditions, in actual fact they produce significantly more power. On the other hand, a nominal 12V battery requires close to an actual 12V depending on battery state of charge. When a non-MPPT charge controller is charging the battery bank, the PV module is frequently forced to operate at a battery voltage that is less than the optimal operating voltage at which the PV module is capable of producing its maximum power. Hence, non-MPPT charge controllers artificially limit power production to a sub-optimal level by constraining the PV array from operating at maximum output power.
A maximum power point tracking (MPPT) charge controller addresses the aforesaid disadvantage of non-MPPT charge controllers by managing the voltage mismatch between the PV array and the battery bank through the use of power electronics. The primary functions performed by MPPT charge controllers involve measuring the PV module output to find the maximum power voltage (Vmp), i.e. the voltage at which the PV module is able to produce maximum power, and operating the PV module at the maximum power voltage to extract or harvest full power (watts) from the PV array, regardless of the present battery voltage (VB).
Photovoltaic modules are made up of photovoltaic (PV) cells that have a single operating point where the values of the current (I) and voltage (V) of the cell result in a maximum power output. The maximum power voltage varies with operating conditions including weather, sunlight intensity, and PV cell temperature. As the maximum power voltage (Vmp) of the PV module varies, the MPPT charge controller “tracks” the Vmp and adjusts the ratio between the maximum power voltage and the current delivered to the battery in order to match what the battery requires. The MPPT charge controller utilizes a control circuit or logic to search for the maximum power output operating point and employs power electronics to extract the maximum power available from a PV module.
A MPPT charge controller employs power electronics that have a higher input voltage than output voltage, hence Vmp>VB. Typically, Vmp is greater than 15V for a 12V nominal battery. The power electronics are conventionally designed to include a high frequency DC to DC converter that receives the maximum power voltage from the PV array as converter input and converts the maximum power voltage to battery voltage as converter output. An increase in battery charge current is realized by harvesting PV module power that would be left unharvested using a non-MPPT charge controller. As the maximum power voltage varies, the actual charge current increase that is realized will likewise vary. Generally speaking, the greater the mismatch or disparity between the PV array maximum power voltage and the battery voltage, the greater the charge current increase will be. The charge current increase will ordinarily be greater in cooler temperatures because the available power output and the maximum power voltage of the PV module increases as the photovoltaic cell temperature decreases. In addition, lower battery voltage, as in the case of a highly discharged battery, will result in a greater charge current increase.
Most MPPT charge controllers utilize power electronics designed to include a “buck” converter having topology to “buck” a higher input voltage to a lower output voltage. Buck converters are familiar in the field of power electronics and essentially include an inductor and two complementary switches to achieve unidirectional power flow from input to output. A first of the switches is ordinarily a controlled switch such as a MOSFET or transistor, and the second of the switches is ordinarily an uncontrolled switch such as a diode. The buck converter alternates between connecting the inductor to the input voltage (VA) from the PV array to store energy in the inductor and discharging the inductor into the battery bank. When the first switch is turned on for a time duration, the second switch becomes reverse biased and the inductor is connected to the input voltage (VA). There is a positive voltage (VL) across the inductor equal to the input voltage (VA) minus the output voltage (VB), hence VL=VA−VB and there is an increase in the inductor current (IL). In this “on” state, energy is stored in the inductor. When the first switch is turned off, inductor current IL continues to flow due to the inductor energy storage, resulting in a negative voltage across the inductor (VL=−VB). The inductor current now flows through the second switch, which is forward biased, and current IL through the inductor decreases. In this “off” state, energy continues to be delivered to the output until the first switch is again turned on to begin another on-off cycle. The buck converter is operated in continuous conduction mode (CCM) when the current through the inductor never falls to zero during the commutation cycle. If the buck converter is operated in continuous conduction mode, the output voltage (VB) is equal to VA×d, where d is the duty cycle (d=[O,1]) of the switches. The buck converter is operated in discontinuous conduction mode (DCM) when the current through the inductor goes to zero every commutation cycle.
Traditional buck converters give rise to input and output ripples, and one approach that has been taken to reduce these ripples involves adding capacitors to the buck converter circuitry for filtering. The input current to the buck converter is discontinuous, being a series of pulses, and it has a very high ripple. In order to limit the resulting voltage ripple around the maximum power voltage, large input capacitors are typically required which must also be rated to handle the ripple current. Although the output current from the buck converter is continuous, output capacitors are still normally employed for filtering to reduce the ripple seen by the battery. The use of capacitors to filter input and output ripples has various disadvantages including added cost and slowing down the system control bandwidth that manages transients.
MPPT charge controllers have been designed with the buck converter implemented as a multi-phase buck converter in which the phases are staggered or interleaved, resulting in reduced input and output ripples. Consequently, capacitor size and cost are reduced, and higher frequency system control bandwidth is made possible. In a two-phase arrangement, the buck converter may be implemented as two smaller buck converter phase configurations in parallel, with each of the buck converter phase configurations having its own inductor and switches. One buck converter phase configuration is run 180° out of phase from the other buck converter phase configuration so that the current pulses assist in cancelling each other with the result that ripple is reduced. In general, the worst case ripple for a single phase buck converter is 50% the output current and for a two-phase buck converter it is 25% the output current. Accordingly, a 2:1 reduction in worst case ripple can be obtained with a two-phase buck converter in comparison with a one-phase or single-phase buck converter. Although the multi-phase arrangement can reduce input and output ripple by a factor of 2, it does not affect the ripple on the internal phase components of the buck converter phase configurations. Accordingly, the ripple is not reduced on the individual components, i.e. the switches and inductor, within each buck converter phase configuration. In the area of MPPT charge controllers for PV systems, it has not previously been recognized to couple the inductors of the buck converter phase configurations in a multi-phase buck converter to reduce ripple currents in the inductors and switches themselves. In the area of MPPT charge controllers for PV systems, various obstacles have stood in the way of a coupled inductor multi-phase buck converter including the increased EMI associated with a coupled inductor, the added difficulty involved in controlling the buck converter as the phases interact, and the loss of stability in simple controller loops.